The present invention relates to a new and novel process for the biological purification of waste water in which an aqueous suspension of the surplus sludge formed during the purification is hydrolyzed in a basic medium and at an elevated temperature.
A similar process is disclosed in Netherlands patent application No. 6,911,163. According to this application the sludge formed is hydrolyzed with the aid of sodium hydroxide or, possibly, calcium oxide. The resulting solid matter which consists essentially of inorganic salts and water-insoluble organic substances such as cell walls which are not, or only partly, hydrolyzed, is filtered off and subjected to further processing. The hydrolysate, which consists essentially of an aqueous solution of mostly organic material and sodium salts, is returned to the purification plant. Although this process produces a great improvement in the disposal of sludge from a water water purification plant, it has disadvantages.
One disadvantage is if sodium hydroxide is used for hydrolysis of sludge. In that event, the hydrolysate will contain large quantities of sodium ions. During subsequent processing of the hydrolysate, which consists essentially of an aqueous solution of amino acids, oligopeptides and hydrolysis products of carbohydrates and fats, these sodium ions present problems. The basic amino acids are the commercially important components of the hydrolysate. If these are to be recovered from the hydrolysate by means, for example, of selective ion exchange, all sodium ions must be removed first.
Another disadvantage is at the temperature and pH applied for the hydrolysis, racemization of the amino acids and peptides may occur. This causes a mixture of D- and L-amino acids to form which is less attractive economically when used, for example, as cattle feed. There is not much point in adding such a mixture of D- and L-amino acids to human food, since D-amino acids have no value as building material. Addition of a considerable amount of D-amino acids is not permitted by law. The only possible alternative is to return it to the waste water purification plant which results in increased investment variable costs.
Moreover, from an environmental consideration, a process of this kind is not attractive anyway because all of the sodium hydroxide used is discharged into the surface water in the form of sodium salts. This causes an extra consumption of chemicals because the hydrolysate, prior to being returned to the plant, must first be neutralized. The pH value and the quantity of liquid are such that interference with the action of the biologically active microorganisms may also occur.
The purpose of the invention is to provide a process in which these disadvantages do not occur.
The process according to the present invention is carried out by giving the suspension of the sludge a pH value of between about 8 and about 11 and carrying out the hydrolysis in the presence of a volatile base at a temperature of between about 90.degree. and about 300.degree.. The base is removed from the hydrolysate formed after the hydrolysis has been terminated. By preference, ammonia or a bound form thereof, such as ammonium carbonate, is used as the volatile base. The ammonia that has been removed from the hydrolysate is returned to the hydrolysis stage. The hydrolysis is, preferably, carried out at a temperature of between about 90.degree. and about 200.degree. C.